The present invention relates, in general, to the production of ultra-short light pulses which are tunable in the infrared, and more particularly, to an externally pumped optical parametric oscillator which provides femtosecond output light pulses at a high repetition rate, the light being in the infrared wavelength band and being potentially tunable in a range from about 1.0 to about 2.75 micrometers.
Laser pulses in the femtosecond (fs) time domain; that is, pulses having widths of from 10.sup.-15 to 10.sup.-13 second, have become important tools for studying extremely short-duration events such as chemical reactions, for by directing femtosecond pulses at reacting chemicals, for example, researchers can capture spectroscopic information about the structure and behavior of short-lived molecular intermediates of the reactions. Femtosecond lasers are also important for studying the dynamics and the ultimate limits of high-speed semiconductor electronic and optical devices. Moreover, numerous ultra-fast phenomena have been inaccessible to direct optical study due to a lack of light sources having appropriate wavelengths or having other suitable characteristics such as tunability or a high repetition rate.
A particularly interesting region of optical study lies in the near- to mid-infrared where, for example, it is possible to study carrier dynamics in most families of semiconductors as well as to do time-domain vibrational spectroscopy in many molecules. Tunable sub-picosecond pulses in various parts of the near- to mid-infra-region have been generated by a number of methods in the past, but such methods have either required amplification of a femtosecond laser followed by supercontinuum generation and seeded parametric amplification, or in one case has required a transient break up of picosecond pulses in a parametric oscillator. However, for many experiments, especially in condensed matter, the low pulse repetition rates of all of these methods, which generally have been in the range of about 10 Hertz, although some have been as high as 8 kHz, as well as the expense associated with the amplified systems, has created numerous problems.
Although optical parametric oscillators have been recently exploited in the femtosecond time domain as a source of broadly and continuously tunable radiation, a lack of suitable pump sources has hampered the development of such devices that operate with short pulses, high repetition rate, and high output power. The high peak powers at the intracavity focus of a colliding-pulse mode-locked dye laser were exploited to develop the first femtosecond optical parametric oscillator, described in U.S. Pat. No. 5,017,806. This resulted in pulses of about 105 fs, with a pulse repetition rate of about 80 MHz at approximately 3 mW output power. Others have resorted to a Q-switched and mode-locked laser (operating at 15 Hz) to pulse an OPO producing pulses having a duration of 160-260 fs (although obtaining 65 fs at one wavelength under special conditions) at 4.5 milliwatts average power (see R. Laenen et al, "Broadly Tunable Femtosecond Pulses Generated by Optical Parametric Oscillation", Optics Letters, Volume 15, No 17, Sep. 1, 1990, pages 971-973). More recently, a femtosecond OPO was reported which was externally pumped by a hybridly mode-locked dye laser, to produce 220 fs pulses at 30 mW average power. However, in order to meet present needs, a tunable source of optical pulses that operate with high output powers and high repetition rate, as well as with very narrow pulse widths is required.